Disc drives are data storage devices used to store and retrieve digital user data in a fast and efficient manner. A typical disc drive stores such data on one or more magnetic recording discs which are rotated at a constant high speed. An actuator moves one or more data transducing heads to access data stored in tracks defined on the disc surfaces.
With the continued demand for disc drives that provide ever higher data storage capacities and transfer rate performance levels at lower costs, designers continue to provide successive generations of products with ever higher areal data storage densities. It will be recognized that for a given area on the recording surface of a disc, more data can be stored by increasing the number of bits per linear distance along the tracks (e.g., bits per inch, BPI) as well as by increasing the number of the tracks on the disc (e.g., tracks per inch, TPI).
Servo data are written to the discs during disc drive manufacturing to define the tracks and to provide head position information for a closed loop servo control circuit. The servo control circuit uses the servo data during both seeking and track following operations.
A well-known sector servo architecture places servo information in sectors that alternate with data sectors in a circumferential direction on a disc. Each servo sector includes a Phase Locked Loop (PLL) field (also referred to as an Adaptive Gain Controller (AGC) field), a Servo Address Mark (SAM) or Servo Index Mark (SIM) field, a track identification (Track ID) field, a servo burst field containing servo burst patterns, and a Repeatable Run Out (RRO) field. Information contained in the servo sectors is processed by a servo demodulator and used to control the position of a recording head with respect to the disc.
Before processing the servo bursts, a servo demodulator first adjusts its parameters, then it detects the Servo Address Mark (SAM) or the Servo Index Mark (SIM), and Track IDs associated with each track, to make sure that the magnetic head is in the vicinity of a particular data track center. Afterwards, it processes the servo bursts to fine tune the location of the recording head and force it to the center of the data track. Finally, it corrects any possible non-zero bias in the system resulting from the effect of Repeatable Run Out (RRO) during read and write operations.
At the beginning of a servo sector, the servo demodulator adjusts its parameters and recovers the possible timing offsets in the system to remove any phase and frequency offsets. For this purpose, servo demodulators contain timing recovery circuitry based on either synchronous or asynchronous sampling of analog signals produced in response to the written PLL field on the disc. In the case of synchronous sampling, this circuitry recovers the correct sampling instants for a sampler that is used to sample the analog signal. In the case of asynchronous sampling this circuitry recovers timing offsets from samples of the analog signal where the sampling instants are generated by a free running clock. After the servo demodulator processes information from the PLL field, the recovered timing offset is fixed for the entire servo sector, and the other fields within the sector are processed with this recovered sampling information. However, as the storage areal densities increase, the effects of timing errors, channel noise, distortions, and nonlinearities also increase, and the length of the PLL field in the servo format may be too short for an acceptable timing recovery performance. Thus longer PLL fields may be needed for an acceptable timing recovery performance. However the use of a longer PLL field reduces the servo format efficiency.
Thus there is a need to increase servo performance without increasing the PLL field length.